Biphenyl-fused 1,2-diphenylcyclobutanes underwent orientation selective bond cleavage in the photosensitized reactions using DCA as a sensitizer or aminium cation radical catalyzed reactions. In recent years much attention has been focused on the electron transfer bond cleavage of cyclobutanes in which cation radicals of cyclobutanes are involved as intermediates to undergo bond cleavage reactions. 2 Photosensitized bond cleavage reactions of 1,2-diary1 substituted cyclobutanes have been studied in detail by Pat and Sakurai et al. 3 who pointed out that the interaction between the two aryl groups through the cyclobutane bond is essential for the occurrence of bond cleavage. Biphenyl-fused cyclobutanes 1 c seem to be good model compounds to establish the structure-reactivity relationship in electron-transfer bond cleavage reactions since they are expected to undergo two different types of cleavage, i.e., path a and path b, depending on the through-bond interaction. We wish to describe here the orientation selective bond cleavage reactions initiated by electron transfer. According to the reported methods, cyclobutanes l,a 0 was synthesized by photochemical (~TI + HIT) cycloaddition of phenanthrene to trans-stilbene, 4 biphenyl (3, respectively.5 The half-wave oxidation % and l&,2 were prepared by thermal and photochemical isomerization of 2,2'-distyryl-0 ph potentials of 12-L were measured by cyclic voltan-metry.